Antenna systems with common overhead for cdma base stations

ABSTRACT

Systems and methods for use in CDMA antenna systems are provided in which signals each having a common overhead component are transmitted on a set of adjacent beams of a sector with a micro-timing offset between signals transmitted on adjacent pairs of beams which is large enough that destructive cancellation substantially does not occur between the pair of beams.

RELATED APPLICATIONS

The application is a CIP of U.S. application Ser. No. 09/733,059 whichwas filed on Dec. 11, 2000. This application also claims the benefit ofU.S. Provisional Application No. 60/423,084 filed Nov. 1, 2002.

FIELD OF THE INVENTION

This invention relates in general to CDMA (Code Division MultipleAccess) cellular communication systems and in particular to methods andapparatus for increasing the capacity of such systems.

BACKGROUND OF THE INVENTION

CDMA cellular systems are currently in widespread use throughout NorthAmerica providing telecommunications to mobile users. In order to meetthe demand for transmission capacity within an available frequency bandallocation, CDMA cellular systems divide a geographic area to be coveredinto a plurality of cell areas. Within each cell is positioned a basestation with which a plurality of mobile stations within the cellcommunicate.

In general, it is desired to have as few base stations as possible. Thisis because base stations are expensive and require extensive effort inobtaining planning permission. In some areas, suitable base stationsites may not be available. In order to have as few base stations aspossible, each base station ideally has as large a capacity as possiblein order to service as large a number of mobile stations as possible.Several key parameters that determine the capacity of a CDMA digitalcellular system are: processing gain, ratio of energy per bit to noisepower, voice activity factor, frequency reuse efficiency and the numberof sectors in the cell-site antenna system.

AABS (Adaptive Antenna Beam Selection) is a method used in CDMA cellularBase Stations to improve traffic capacity in “hot spot” sectors withoutrequiring additional carriers (i.e. more spectrum) at the hot spot. Thisspectrally efficient technique replaces the standard sector antenna beampattern with a multiplicity, typically three, of beams per sector. Thesenew beams have higher directivity on both the forward and reverse links.This higher directivity reduces the forward interference seen by amobile terminal and reduces the received interference level at the basestation's receiver. Consequently, the RF power required to support atypical call in the forward ink is lower than that required for aconventional antenna beam. This results in a significantly greaternumber of AABS calls being supportable with a base station's limitedtransmitter power than is possible with a conventional sector beam.

In a similar manner to the forward link situation, the reverse link AABSbeams are more directive than a conventional sector beam. As a result,the mobile terminal's RF power required to support a typical call in anAABS sector will be lower than for a conventional sector call. This willalso help prolong the mobile terminal's battery life.

An example of the AABS method of achieving an increase in capacity isshown in FIG. 1 in which a single wide beam width antenna per sector isreplaced with an antenna array that allows the formation of a number ofnarrower beam widths that cover the area of the original beam. Referringto FIG. 1, a conventional CDMA communication cell 100 is showncomprising three adjacent sectors, alpha 102, beta 104 and gamma 106.Each cell comprises an antenna tower platform 120 located at theintersection of the three sectors. The antenna tower platform 120 hasthree sides forming an equilateral triangle. Each sector has threeantennas. Only the antennas in sector alpha 102 are shown, and theseconsist of a first antenna 114, a second antenna 116 and third antenna112 mounted on a side of the antenna tower platform 120. Each sectoralso has three beams. Only the beams in sector alpha 102 are shown, andthese consist of a first beam 108, a second beam 110 and a third beam112. The three beams 108,110,112 are adjacent with some overlap. Thethree sectors alpha 102, beta 104 and gamma 106 are identical instructure with respect to antennas and beams. The signal for aparticular user can then be sent and received only over the beam orbeams that are useful for that user. If the pilot channel on each beamis unique (i.e. has a different PN (pseudo-random noise) offset) withineach sector then the increase in capacity is limited due to interferencebetween reused pilot channels in different cells.

An improvement is to use multiple narrow beams for the traffic channelsand transmit the overhead channels (pilot, sync, and paging channels)over the whole sector so that the pilot channel is common to all thenarrow beams used by the traffic channels in that sector. This leads tosubstantial gains in capacity. For example, a change from a system witha single beam per sector to a system with three beams per sector with acommon pilot channel yields a 200 to 300% increase in capacity. It istherefore desirable that the pilot channel be broadcast over the areacovered by the original wide beam. A possible arrangement is to usemultiple beams per sector for the traffic channels and transmit theoverhead channels over a separate wide beam antenna covering the wholesector. However, this requires the expense of extra hardware as well asthe calibration and adjustment needed to match the phase of the pilotchannel with the phase of the traffic channels over time andtemperature.

Another possible solution is to use adaptive antenna array techniques totransmit and receive multiple narrow beams for the traffic channels andto transmit the overhead channels over the whole sector on the sameantenna array. However, this requires complex calibration equipment andalgorithms.

Yet another solution is to use an antenna array that transmits andreceives multiple sectors over fixed narrow beams for the trafficchannels and transmits the pilot channel on the same fixed narrow beams.However, the problem with this approach is that the strength of thepilot channel signal at any point in the sector is determined by thevector sum of all of the pilot channel signals from each beam. Since thepilot channel signals from each beam are coherent, areas where thevector sum of the pilot channel signals is null or severely degradedwill occur. This can result in dropped calls when a mobile stationenters one of these areas.

There is thus an advantage to provide an antenna array that uses fixednarrow beams for transmitting and receiving the traffic channels onmultiple beams and can broadcast the common pilot channel over all ofthe sector using the same antenna array. Furthermore, it would beadvantageous to provide an antenna system that did not require complexcalibration and adjustment to maintain performance over time andtemperature.

In CDMA cellular systems the (typically three) sectors at each site areidentified by their PN offsets of the short code. The short code is afull length pseudo random sequence of 2̂15-1 bits which repeats exactly75 times every two seconds. All PN offsets are differentiated from eachother by multiples of 64 chips which results in 512 PN offsets in total.In a similar manner to frequency re-use planning in a frequency divisionnetwork, a CDMA network needs to have a PN plan which avoids PNambiguities between different sectors and an effect known as PNpollution which can seriously impact a network's traffic capacity anddropped call rate.

Bearing in mind the limited number of PN offsets available to a network,one of the major advantages of AABS is that the increased number ofbeams utilized by the AABS sectors does not increase the number of PNoffsets required by the network. Consequently there is no requirementfor expensive and time consuming “optimization” when AABS sectors areintroduced into a CDMA network.

SUMMARY OF THE INVENTION

Other aspects and features of the present invention will become apparentto those ordinarily skilled in the art upon review of the followingdescription of the specific embodiments of the invention in conjunctionwith the accompanying figures.

According to one broad aspect, the invention provides an antenna systemfor a transmitter comprising: a plurality of antennas defining arespective plurality of fixed beams which together cover a coveragearea; for each antenna a respective signal generator generating arespective signal comprising a common overhead component common to allthe signals; transceiver circuitry connecting the signal generators tothe antennas such that a respective one of the signals is transmitted byeach antenna; wherein the each pair of signals transmitted on anadjacent pair of said antennas has a respective mutual micro-timingoffset which is large enough that destructive cancellation substantiallydoes not occur between the pair.

In some embodiments, an antenna system, implemented for a plurality ofcoverage areas, each coverage, area being a respective sector served bythe base station.

In some embodiments, the transmitter is a CDMA base station, and eachsignal is a CDMA signal.

In some embodiments, the respective mutual micro-timing offset is smallenough that substantially no signal source ambiguity occurs at areceiver.

In some embodiments, the sector has a sector-specific spreading code,and wherein the respective mutual micro-timing offset between each pairof CDMA signals is realized by applying the sector-specific spreadingcode with a respective mutual micro-offset.

In some embodiments, the sector-specific spreading code is a PN code.

In some embodiments, each mutual micro-offset is at least one chip andless than eight chips.

In some embodiments, each mutual micro-offset is half a width of atraffic search less than a window/space implemented in a mobile terminalcommunity with the base station.

In some embodiments, the sector-specific code is a short code having asector specific offset used to distinguish between other sources usingthe same short code, and wherein the respective mutual micro-timingoffset is small enough that substantially no ambiguity between differentsector specific offsets occurs at a receiver in respect of any pair ofsignals transmitted by adjacent antennas.

In some embodiments, the short code is of length 2̂15-1.

In some embodiments, the sector has a sector-specific spreading code,and wherein the respective mutual micro-timing offset between each pairof CDMA signals is realized by applying the micro-timing offset torespective sector-specific spreading code generators.

In some embodiments, the common overhead component comprises at leastone of pilot channel, sync channel, paging channel, quick paging,advanced access channel and auxiliary pilot.

In some embodiments, a system further comprises: for each active userlocated within the sector, at a given instant only one of the CDMAsignals includes a user-specific traffic component generated by therespective CDMA signal generator.

In some embodiments, the one of the CDMA signals to include theuser-specific traffic component for a given user is identified byanalyzing signal strength on reverse links from the user, and selectingthe CDMA signal corresponding with the reverse link having a best signalstrength.

In some embodiments, the transceiver circuitry is further adapted toprovide transmit frequencies in a manner such that the transmitfrequencies include a frequency offset from one another.

In some embodiments, a system comprises a beam-forming matrix.

In some embodiments, the beam-forming matrix is a Butler matrix.

In some embodiments, the frequency offset is chosen to further reduceundesirable effects of signal cancellation.

In some embodiments, the signals have unique traffic channels.

In some embodiments, the frequency offset is a multiple other than thatof the frame rate.

In some embodiments, the frequency offset is greater than 30 Hz and lessthan 120 Hz.

In some embodiments, a system further comprises: means in thetransceivers for providing transmit phases that include a time dependentphase offset from one another, wherein the phase offset is chosen toreduce undesirable effects of signal cancellation.

According to another broad aspect, the invention provides a method in aCDMA antenna system comprising transmitting signals each having a commonoverhead component on a plurality of adjacent beams of a sector with amicro-timing offset between signals transmitted on adjacent pairs ofbeams which is large enough that destructive cancellation substantiallydoes not occur between the pair of beams.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a conventional tri-cellular CDMA communicationcell modified to show three narrow beams in place of the normal singlewide beam per sector;

FIG. 2A is a diagram of an antenna system provided by an embodiment ofthe invention;

FIG. 2B is a diagram of another antenna system provided by an embodimentof the invention;

FIG. 3 is a diagram showing a transceiver provided by an embodiment ofthe invention which may be used in the antenna systems of FIGS. 2A and2B; and

FIGS. 4A and 4B are diagrams showing the vector addition of signals froma first beam and a second beam.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In order to transmit and receive unique traffic channels on each beam ina sector while transmitting common overhead channels (for example pilot,sync, paging channels, quick paging, advanced access channel andauxiliary pilot) over all of the beams in the sector, an antenna systemusing fixed narrow beams that does not require complex calibrationequipment and algorithms is provided.

To this end, FIG. 2A shows an antenna system 200 that is implementedwithin one sector of a sectorized base station, for example withinsector alpha 102 of FIG. 1. The sectors beta 104 and gamma 106 haveidentical antenna systems. The antenna system 200 defines a first beam108, a second beam 110 and a third beam 112. The three beams 108,110,112are radiation/reception patterns formed by a first antenna 114, a secondantenna 116 and a third antenna 118 respectively. The three antennas114,116,118 are connected to a beam-forming matrix 240 that may be, forexample, a Butler matrix. The beam-forming matrix 240 comprises threebi-directional ports: a first port 242, a second port 244 and a thirdport 246. The input signals of the first port 242, the second port 244and the third port 246 are transmitted on the first beam 108, the secondbeam 110 and the third beam 112 respectively. The signals received onthe first beam 108, the second beam 110 and the third beam 112 are theoutputs of the first port 242, the second port 244 and the third port246 respectively. The antenna system 200 also comprises a firsttransceiver 220, a second transceiver 222 and third transceiver 224. Thefirst transceiver 220 has an input 226, an output 228 and abi-directional port 252. The second transceiver 222 has an input 230, anoutput 232 and a bi-directional port 254. The third transceiver 224 hasan input 234, an output 236 and a bi-directional port 256. The firstport 242, second port 244 and third port 246 of the beam-forming matrix240 are connected to bi-directional port 252 of the first transceiver220, bi-directional port 254 of the second transceiver 222 andbi-directional port 256 of the third transceiver 224 respectively.

Also shown for each transceiver 220,222,224 is a respective CDMA signalgenerator 219,221,223 each of which generates a respective one of theinput signals 226,230,234 to the transceivers 220,222,224. The detailsof the CDMA signal generators 219,221,223 are not shown. This is becausethere are many different ways to implement circuits/systems whichgenerate CDMA signals which are well understood. The salient feature isthat each signal is generated using a respective PN code. The PN code isunique to each sector (at least it is unique within some area). Adifferent PN code would be generated for each sector. Nominally, thesame PN code is transmitted on all of the three narrow beams108,110,112. However, in this embodiment of the invention, a small timeoffset is applied to the PN code used in two of the three CDMA signalgenerators, the effect of this being that the first signal 226 isgenerated with a PN code PN(t), the second signal 230 is generated usingthe PN code PN(t+Δt₁) and the third signal 234 is generated with a PNcode PN(t+Δt₂). In other words, the PN code used to generate the secondsignal 230 is offset from that used to generate the first signal 226 bya time offset Δt₁. Similarly, the PN code used to generate the thirdsignal 234 is offset from that used to generate the second signal 230 bya time offset of Δt₃=Δt₂−Δt₁. Each PN-code generator may for example beimplemented with a series of flip-flops with specific interconnects.Different time offset can be achieved by starting the flip-flops at apredefined state at t=0. Different time-shifts require differentstarting defined states. There are many other ways of PN code generationwhich would be understood by one skilled in the art. The offsets Δt₃,Δt₁ are selected to reduce substantially interference overhead portionstransmitted on overlapping beams in a single sector.

The example of FIG. 2A includes three beams per sector. The invention isin no way limited to this. More generally, it is applicable for anynumber of beams greater than two. The mutual micro-timing offset is thenintroduced between each pair of adjacent beams. What is important isthat adjacent beams have a timing offset which reduces the interferencein the overlap portions. The overall set of offsets should fall withinthe search window of the receiver. This may necessitate some positiveoffsets and some negative offsets if the number of beams is large. Forexample, even numbered beams could use p(t) and odd numbered beams coulduse p(t+Δt).

The Algorithm

Each beam is offset from its neighbouring beam by a “micro” PN offset.Micro in this context means at least a minimum resolvable PN chip offset(for example 814 nS for IS-95 and 1XRTT), but less than a maximum whichmight introduce base station identification ambiguities in a network.

The minimum micro PN offset is a system specific parameter. Preferablyit is just large enough so that substantially no destructiveinterference occurs in the area of beam overlap. The minimum may also befurther defined by the minimum offset differential which can beindividually discriminated at the receiver. In one example, this minimumis set to one chip. The main reason for applying this micro-PN offset isto avoid destructive interference in the beam overlap region.

The second requirement for an offset that is low enough to avoid PNambiguities from other base stations is necessary to minimize pilotpollution effects such as dropped calls and excess required forwardpower. In systems which do not employ PN offsets for signal sourceidentification, this maximum “micro” offset constraint may not benecessary. The exact value of this maximum PN offset would likely benetwork specific and proportional to the PN increment used in thenetwork, the “PN increment” being a multiple of the system PN offset (64chips for many systems). For example, for networks with PN increments of4, an 8 chip differential offset would be a practical maximum whereas indense urban environments with a PN increment of 2, a 4 chip maximumdifferential offset would be more appropriate.

A third implementation specific consideration in selecting the micro PNoffset is the search window signal used by mobile terminals. In orderthat the mobile terminal will find the different signals with itssearcher, it is preferred that the set of mutual micro-timing offsets isless than half a width of a traffic search window implemented in amobile terminal communicating with the base station.

An advantage of this scheme from a forward link power point of view, isthat the destructive cancellation between forward link signals isavoided without adding artificial fading (described below), thus leadingto an improved reliability and tolerance to beam selection errors in thebeam overlap regions.

FIG. 2B shows another antenna system 202 that may be deployed forexample within sector alpha 112, as provided by an embodiment of theinvention. The antenna system 202 defines a first beam 108, a secondbeam 110 and a third beam 112. The three beams 108,110,112 areradiation/reception patterns formed by a first antenna 114, a secondantenna 116 and a third antenna 118 respectively. The antenna system 202also comprises a first transceiver 220, a second transceiver 222 andthird transceiver 224. The first transceiver 220 has an input 226, anoutput 228 and a bi-directional port 252. The second transceiver 222 hasan input 230, an output 232 and a bi-directional port 254. The thirdtransceiver 224 has an input 234, an output 236 and a bi-directionalport 256. The three antennas 114,116,118 are connected to the threerespective bi-directional ports 252,254,256 of the transceivers220,222,224. The antenna system 202 also comprises a digital beam former260 that has a first input 262, a first output 264, a second input 266,a second output 268, a third input 270, a third output 272, a first beamoutput 274, first beam input 276, a second beam output 278, a secondbeam input 280, a third beam output 282 and a third beam input 284. Thefirst beam output 274 and input 276 of the digital beam former 260 areconnected the input 226 and output 228 of the first transceiver 220respectively. The second beam output 278 and input 280 of the digitalbeam former 260 are connected the input 230 and output 232 of the secondtransceiver 222 respectively. The third beam output 282 and input 284 ofthe digital beam former 260 are connected the input 234 and output 236of the third transceiver 220 respectively.

Although three antennas forming three beams per sector are used in thisexample of the preferred embodiment, any number of antennas and beamsper sector greater than one may be used while remaining within the scopeof the invention.

Referring to FIG. 2B, the signals on input 226, input 230 and input 234of transceiver 220, transceiver 222 and transceiver 224 respectively aredigital baseband signals that are transmitted on the first beam 108, thesecond beam 110 and the third beam 112 respectively. The signals onoutput 228, output 232 and output 236 of transceiver 220, transceiver222 and transceiver 224 respectively are digital baseband signals thatare received on the first beam 108, the second beam 110 and the thirdbeam 112 respectively.

The digital baseband signals on input 226, input 230 and input 234 oftransceiver 220, transceiver 222 and transceiver 224 respectively may beany CDMA standard digital data stream adapted to be received by aplurality of mobile stations (not shown) within the area covered by thefirst beam 108, the second beam 110 or the third beam 112.

Similarly, referring to FIG. 2B, the signals on input 262, input 266 andinput 270 of the digital beam former 260 are digital baseband signalsthat are transmitted on the first beam 108, the second beam 110 and thethird beam 112 respectively. The signals on output 264, output 268 andoutput 272 of the digital beam former 260 are digital baseband signalsthat are received on the first beam 108, the second beam 110 and thethird beam 112 respectively.

Also shown are CDMA signal generators 219,221,223 which function in thesame manner as the like-numbered elements of FIG. 2A.

The digital baseband signals on input 262, input 266 and input 270 ofthe digital beam former 260 may be any CDMA standard digital data streamadapted to be received by a plurality of mobile stations (not shown)within the area covered by the first beam 108, the second beam 110 orthe third beam 112.

The transceivers 220,222,224 of FIG. 2A or FIG. 2B may be conventionaltransceivers, or may be designed so as to further reduce the probabilityof destructive interference occurring in the area of overlap adjacentnarrow beams.

FIGS. 2A and 2B provide two examples of transceiver circuitries betweenthe CDMA signal generators and the antennas. Other transceivercircuitries can be employed to get the CDMA signals transmitted on thebeams.

Another implementation of the transceivers of FIGS. 2A and 2B will nowbe described with reference to FIG. 3. This embodiment featuresadditional measures beyond the above-described “micro” PN offset whichmay be included in some embodiments. Conventional transceivers could beemployed, and would still benefit from the “micro” PN offset scheme. Forease of description the transceiver shown in FIG. 3 is given thereference number 300. Transceiver 300 has its input 302 connected to aninput of a modulator 306. The modulator 306 has an output 308 that isconnected to a first input 310 of an up-converter 312. The up-converter312 also has a second input 314 and an output 316. The second input 314of the up-converter 312 is connected to an oscillator 318 that may be,for example, a digital frequency synthesizer. The output 316 of theup-converter 312 is connected to an input 344 of a duplexor 340 having abi-directional port 342 connected to the bi-directional port 320 of thetransceiver 300. The transceiver 300 also has an output 322 connected toan output 324 of a demodulator 326. The demodulator also has an input328 that is connected to an output 330 of a down-converter 332. Thedown-converter 332 also has a first input 334 and a second input 336.The first input 334 of the down-converter 332 is connected to anoscillator 338 and the second input 336 of the down-converter 332 isconnected to an output 346 of the duplexor 340. The up-conversion stageof the transceiver 300, comprising the up-converter 312 and oscillator318, are shown as a single stage for convenience. In reality theup-conversion may be done in a plurality of stages. Similarly, thedown-conversion stage of the transceiver 300, comprising thedown-converter 332 and oscillator 338, are shown as a single stage forconvenience. In reality the down-conversion may be done in a pluralityof stages.

The frequency of the oscillator 318 in transceiver 222 is chosen suchthat the frequency of the output 316 of the up-converter 312 in thetransceiver 222 is a standard base station transmit frequency, f_(c).The frequency of the oscillator 318 in the transceiver 220 is chosensuch that the frequency of the output 316 of the up-converter 312 in thetransceiver 220 is f_(c) plus an offset frequency, f_(o). The frequencyof the oscillator 318 in the transceiver 224 is chosen such that thefrequency of the output 316 of the up-converter 312 in the transceiver224 is f_(c) minus the offset frequency, f_(o). For example, iff_(c)=1940 MHz and f_(o)=40 Hz, then the frequency output of theup-converter 312 in the transceiver 222 equal to 1940 MHz, the frequencyoutput of the up-converter 312 in the transceiver 220 is equal to1940.00004 MHz and the frequency output of the up-converter 312 in thetransceiver 224 is 1939.99996 MHz. The purpose of including theseoffsets in the oscillator frequencies used in the transceivers is toreduce the likelihood that in the area of overlap between adjacent beamsthere will be destructive interference. In the example given above,adjacent pairs of frequencies used for the three beams are spaced by anidentical 40 Hz. However, it is to be understood that it is notessential to use identical spacing between adjacent pairs.

The signal strength of the pilot channel at any point in the sector isdetermined by the vector sum of all of the pilot channel signals fromeach beam. For example, referring to FIG. 4A, the signal at a point fromthe second beam 110 is represented by vector 402. The signal at the samepoint from the first beam 108 is represent by vector 404. Since thefrequency of the signal transmitted on the first beam 108 is offset byf_(o) from the frequency of the second beam 110, the vector 404 rotateswith respect to vector 402 and hence, the magnitude of resultant vector406 will fluctuate with a 1/f_(o) time period. FIG. 4B shows a plot ofthe magnitude 408 of the result vector 406 versus time 410. Due to therotation of vector 404 a minimum 416 value occurs every 1/f_(o) 414. Inan IS-95 forward channel, the frame rate f_(f) is 50 frames per secondor a period of 20 ms. Also, each IS-95 frame is repeated once. Thereforethe offset frequency f_(o) is chosen such that 1/f_(o) 414 is not amultiple of 1/f_(f) 412. This will prevent a minimum 416 from occurringat the same point in two consecutive frames thus significantly reducingthe error rate.

Since the magnitude of the resultant vector 406 fluctuates with a1/f_(o) time period, f_(o) is chosen by empirical methods such that theoverall system performance is optimized. The optimum value of f_(o), foreach base station, is influenced by environmental factors, the maximumvelocity of the mobile stations, the frequency band and the over-the-airinterface. Typically f_(o) is greater than 30 Hz and less than 120 Hzfor an IS-95 CDMA communication system. Other over-the-air interfacestandards may have optimum performance at different values of f_(o).

The frequencies of oscillator 338 in transceiver 220, oscillator 338 intransceiver 222 and oscillator 338 in transceiver 224 are identical andchosen such that IS-95 signals at standard frequencies aredown-converted and demodulated.

The traffic channels on each beam are unique and uncorrelated so that nocancellation of the traffic channels occurs.

In an alternative embodiment, the waveform of the oscillator 318 intransceiver 222 is chosen such that the waveform of the output 316 ofthe up-converter 312 in the transceiver 222 is a standard IS-95 basestation transmit frequency, f_(c). The waveform of the oscillator 318 inthe transceiver 220 is chosen such that the waveform of the output 316of the up-converter 312 in the transceiver 220 is f_(c) with a timedependent phase offset within a range of −180° to 180°. The waveform ofthe oscillator 318 in the transceiver 224 is chosen such that thewaveform of the output 316 of the up-converter 312 in the transceiver224 is f_(c) with time dependent phase offset within a range of −180° to180°. The waveform of the output 316 of the up-converter 312 in thetransceiver 222 is the reference for 0° phase. The time dependent phaseoffset within may be sinusoidal, random or any other pattern thatresults in the phases of the output of oscillator 318 in the transceiver220, the output of oscillator 318 in the transceiver 222 and the outputof oscillator 318 in the transceiver 224 being incoherent. Hence, thephases of the first beam 108, the second beam 110 and the third beam 112are incoherent.

In the preferred embodiment the signals on input 226, input 230 andinput 234 of transceiver 220, transceiver 222 and transceiver 224respectively have identical IS-95 overhead channels (pilot,synchronization and paging channels) and unique IS-95 traffic channelscorresponding to mobile station(s) (not shown) that aretransmitting/receiving on the first beam 108, the second beam 110 andthe third beam 112 respectively. Mobile stations that move from beam tobeam or are in an area of overlapping beams are handled by IS-95 handoffprocedures.

In an alternative embodiment the signals on input 226, input 230 andinput 234 of transceiver 220, transceiver 222 and transceiver 224respectively have identical IS-2000 overhead channels and unique IS-2000traffic channels corresponding to mobile station(s) (not shown) that aretransmitting/receiving on the first beam 108, the second beam 110 andthe third beam 112 respectively. Mobile stations that move from beam tobeam or are in an area of overlapping beams are handled by IS-2000handoff procedures.

It should be noted that while an embodiment of the invention using aButler matrix 240, as shown in FIG. 2A, does not require a calibrationscheme to compensate for differential phases between the transceivers,an embodiment using a digital beam former 260, as shown in FIG. 23, doesrequire a calibration scheme to compensate for differential phasesbetween the transceivers.

Advantageously, the invention may be used with antenna systems employingdiversity schemes, such as space diversity or polarization diversity. Inall diversity schemes all overlapping beams should have offsetfrequencies or time dependent phase offsets.

The above embodiments have focussed on IS-95 CDMA implementations. Moregenerally, these techniques are applicable wherever a sector-specificspreading code is applied within a sector within which multiple beamsare being transmitted. PN codes with micro-PN offsets are the sectorspecific codes employed in the described embodiments. More generally,sector-specific codes with micro-offsets may be employed. A micro-offsetis to be distinguished from a regular offset which might be used toindicate two different codes (the same code separated in time by theregular offset—64 chips for the examples described). The invention mayalso be applicable to non-CDMA systems.

The above described embodiments have solved the overlap problem byintroducing a micro-offset in the PN code generation step therebyproducing signals which except for exceptional circumstances will notinterfere destructively within the overlap region of adjacent beams. Inthese embodiments, the timing of the PN Code generation is also tied tothe timing of the signal. This means that if a micro PN offset isapplied, the signal is delayed by the same amount. More generally, thesignals to be transmitted on adjacent beams can be differentiallydelayed as a whole any time after PN code application to substantiallythe same effect.

While the preferred embodiment of the present invention has beendescribed and illustrated, it will be apparent to persons skilled in theart that numerous modifications and variations are possible. The scopeof the invention, therefore, is only to be limited by the claimsappended hereto.

1-27. (canceled)
 28. A multi-beam antenna system, comprising: at leasttwo antennas defining at least two respective antenna beams which form acoverage area; at least one signal generator operable to generate arespective signal for each of the at least two antennas, each respectivesignal comprising a common part and a unique part, the common part beingcommon to the at least two signals and using a coverage area spreadingcode; transceiver and beam-forming circuitry operable to couple the atleast one signal generator to the antennas, the respective signals to betransmitted substantially simultaneously; and for adjacent antennas, therespective common part having a micro-timing offset applied to thecoverage area spreading code of one antenna relative to the coveragearea spreading code of the other antenna of the at least two antennas,the micro-timing offset being large enough that destructive cancellationsubstantially does not occur between the respective signal common partstransmitted on the respective adjacent beams.
 29. A system according toclaim 28, wherein the common part comprises at least one of a pilotchannel, a sync channel, a paging channel, a quick paging channel, anadvanced access channel and an auxiliary pilot channel.
 30. A systemaccording to claim 29, wherein the unique part comprises at least onetraffic channel.
 31. A system according to claim 28, wherein thebeam-forming circuitry is a Butler matrix.
 32. A system according toclaim 28, wherein the beam-forming circuitry is a digital beam former.33. A system according to claim 28, wherein the multi-beam antennasystem employs a diversity scheme.
 34. A system according to claim 33,wherein the diversity scheme is a space diversity scheme.
 35. A systemaccording to claim 33, wherein the diversity scheme is a polarizationdiversity scheme.
 36. A system according to claim 28, wherein oddnumbered beams have a time offset relative to the even numbered beams.37. A system according to claim 28, wherein the signals are IS-95signals.
 38. A system according to claim 28, wherein the signals areIS-2000 signals.
 39. A system according to claim 28, implemented for aplurality of coverage areas, each coverage area being a respectivesector served by a base station, wherein the plurality of fixed beamstogether cover a corresponding one of the sectors, and wherein thesectors are associated with respective different spreading codes.
 40. Asystem according to claim 28, wherein the system is for a transmitter,the transmitter being a CDMA base station, and wherein each signal is aCDMA signal.
 41. A system according to claim 39, wherein the system isfor a transmitter, the transmitter being a CDMA base station, andwherein each signal is a CDMA signal.
 42. A system according to claim28, wherein the coverage area is a cell sector, wherein the respectivemutual micro-timing offset is less than a predefined maximum value suchthat the mutual micro-timing offset does not cause a source of one ofthe signals to be incorrectly identified as located in another cellsector.
 43. A system according to claim 42 wherein: the sector has asector-specific spreading code, and wherein the respective mutualmicro-timing offset between each pair of signals is realized by applyingthe sector-specific spreading code with a respective mutual micro-timingoffset.
 44. A system according to claim 43, wherein the sector-specificspreading code is a PN code.
 45. A system according to claim 44, whereineach mutual micro-timing offset is at least one chip and less than eightchips.
 46. A system according to claim 43, wherein the sector-specificspreading code is a short code having a sector specific offset used todistinguish between other sources using the same short code, and whereinthe respective mutual micro-timing offset is small enough thatsubstantially no ambiguity between different sector specific offsetsoccurs at a receiver in respect of any pair of signals transmitted byadjacent antennas.
 47. A system according to claim 46, wherein the shortcode is of length 2̂15-1.
 48. A system according to claim 41 wherein: thesector has a sector-specific spreading code, and wherein the respectivemutual micro-timing offset between each pair of CDMA signals is realizedby applying the sector-specific spreading code and then applying amutual micro-timing offset.
 49. A system according to claim 41 wherein:the sector has a sector-specific spreading code, and wherein therespective mutual micro-timing offset between each pair of CDMA signalsis realized by applying the micro-timing offset to respectivesector-specific spreading code generators.
 50. A system according toclaim 28, wherein the transceiver circuitry is further adapted toprovide transmit frequencies in a manner such that the transmitfrequencies include a frequency offset from one another.
 51. A systemaccording to claim 50 wherein the frequency offset is chosen to furtherreduce undesirable effects of signal cancellation.
 52. A systemaccording to claim 51, wherein the frequency offset is a multiple otherthan that of a frame rate.
 53. A system as defined in claim 28, whereinthe transceiver circuitry is operable to provide transmit phases thatinclude a time dependent phase offset between transmit phases, whereinthe phase offset is chosen to reduce undesirable effects of signalcancellation.
 54. A method of operating a multi-beam antenna system, themethod comprising: for each antenna of at least two adjacent antennasdefining at least two respective antenna beams which form a coveragearea, generating a respective signal comprising a common part and aunique part, the common part using a coverage area spreading code andthe common part of one generated signal having a micro-timing offsetapplied to the coverage area spreading code relative to the common partof the other generated signal, the micro-timing offset being largeenough that destructive cancellation substantially does not occurbetween the respective signal common parts transmitted on the respectiveadjacent antennas; and coupling the respective generated signals to theantennas, for substantially simultaneous transmission.
 55. A methodaccording to claim 54, wherein the common part comprises at least one ofa pilot channel, a sync channel, a paging channel, a quick pagingchannel, an advanced access channel and an auxiliary pilot channel. 56.A method according to claim 55, wherein the unique part comprises atleast one traffic channel.
 57. A method according to claim 54,comprising employing a diversity scheme for the multi-beam antennasystem.
 58. A method according to claim 57, wherein the diversity schemeis a space diversity scheme.
 59. A method according to claim 57, whereinthe diversity scheme is a polarization diversity scheme.